A typical photographic copying machine is provided with a transparent platen which supports a document during the copying process. The copying machine includes an optical system and various processing means for projecting an image of the graphic information on the original document to a photoconductive member thereby creating a latent electrostatic image of the document information on the photoconductive member. By operation of suitable instrumentalities, the latent image is developed on the photoconductive member by the application of colored marking material, which material either permanently adheres to the photoconductive member or is transferred to another support material, usually plain paper, depending on whether the photoconductive member is in the form of copy paper coated with a photoconductive substance which constitutes the final copy of the original document or is an intermediate, reusable photoconductive member from which the developed image is transferred to ordinary paper. In either type of machine, the original document is illuminated by a source of high intensity light in order to expose the photoconductive member and to cause discharge of the member in the light struck areas to develop the latent electrostatic image on the member. Typical techniques for illuminating the document are scanning, which involves relative movement between the original document and the source of illumination, or by full-frame high intensity flash exposure of the document while both the document and the light source remain stationary even though the photoconductive member may be moving at the instant of the flash of illumination.
Regardless of the type of photoconductive member utilized in the copy machine and regardless of the illumination technique utilized in either type of machine, it is necessary to shield the eyes of the operator of the copy machine from rays of light from the illuminating source during the exposure portion of the copying cycle. Typically, the transparent platen on which the document is placed is dimensioned to be slightly larger than the maximum size document which the copying machine is capable of reproducing, thereby resulting in a small amount of light being able to pass beyond the borders of the document. In addition, the principal use of most copying machines is in the reproduction of documents which are smaller than the maximum size document which the copying machine can reproduce with the result that a considerable amount of light can pass beyond the borders of most documents being copied. Since the degree of illumination is quite intense, to the point of being momentarily blinding in the case of flash illumination copying machines, it is necessary to provide a cover for the transparent platen which will effectively prevent any direct rays of light from the illuminating source from striking the eyes of the operator.
Another important reason for providing a cover for the transparent platen is to provide a reflecting surface for the rays of light passing beyond the borders of the document being copied so that all portions of the photoconductive member within the optical field of the copying machine will be lightstruck in order to discharge the photoconductive member in these areas. This will prevent the deposition of any marking material on the photoconductive member except in the image area so that nothing will appear on the copy sheet except the image of the document even though the document may be smaller than the copy sheet.
Over the years, many different types of document covers have been developed and marketed for copying machines having transparent document receiving platens. One early form of cover which has been used extensively is simply a flexible, usually rubber, mat connected to the housing of the copying machine adjacent one edge of the transparent platen and provided with a handle adjacent the opposite edge by which the cover can be lifted by the operator with one hand while a document is placed on the transparent platen, after which the cover is replaced over the document. The cover is provided with a white undersurface in order to reflect any light passing beyond the borders of the document. Although this form of cover functioned fairly well initially, it had several drawbacks and its useful life was substantially limited. This type of cover is generally hinged along the short edge of the glass platen and, in order to remove it, it is necessary for the operator to grasp the opposite short edge and swing the cover a considerable distance along the long edge of the transparent platen in order to insert a document. If an operator had a large number of documents to copy, it soon became very tiring for the operator to continuously swing the cover from a closed to an open position and back for each document. To avoid this, operators would operate the machine with the cover left open, which is detrimental to the operator's vision and also results in considerable deposition of marking material on the portions of the photoconductor within the optical field of the copying machine but beyond the borders of the document, which considerably shortens the life of a reusable photoconductive member and produces undesirable border markings in the case of the photoconductive copy paper when a small document is being copied. Another disadvantage of the rubber mat type platen cover is the fact that the rubber begins to wear out and crack, particularly along the portion connected to the housing of the copying machine after many thousands of flexures in the course of use of the copying machine. This necessitates frequent change of covers. A further disadvantage is that the white undersurface of the cover soon becomes permanently soiled due to dirt particles becoming embedded in the surface material as the result of continuous flexure of the platen cover. This results in poor copy quality, or "background", since the dirt on the underside of the platen cover is reproduced as part of the image on the copy where the undersurface of the cover is exposed to the photoconductive member beyond the borders of the document.
Partly as a result of these, as well as other, disadvantages of the flexible platen covers, rigid covers were developed which provided certain advantages over the flexible type covers in that they could be manipulated more easily by the operator of the copy machine, they were not as susceptible to becoming soiled due to lack of continuous flexure, and they have proved to be much longer lasting in operation. Typical of such covers are those illustrated in U.S. Pat. Nos. 3,615,134; 3,642,371 and 3,813,161.